Conventionally, so-called tandem-type image forming apparatuses are known. This type of image forming apparatus includes a plurality of photosensitive members for each color (yellow, magenta, cyan, and black, for example) that is arranged in a direction in which a paper conveying belt moves. Images in each color borne on a corresponding photosensitive member are sequentially transferred onto a paper on the belt.
In such a tandem-type image forming apparatuses, if image forming positions on paper for each color are deviated (shifted) from the correct positions, color images with color registration errors are formed undesirably. Hence, one of these image forming apparatuses has a function to correct image forming positions of each color (Japanese Patent Application Publication No. 2007-232763). When performing this correcting function, first the image forming apparatus forms a registration error detection pattern (calibration pattern) on the belt. The registration error detection pattern includes marks in a left group formed along a left end of the belt and marks in a right group formed along a right end of the belt. The both groups have the same configuration where marks in each color are arranged with predetermined spaces along a direction in which the belt moves. The positions of the marks in each group are detected by an optical sensor. Then, amounts of registration errors of respective colors (yellow, magenta, and cyan, for example) relative to a reference color (black in this example) are calculated. A left and right average amount of registration errors are obtained from the amounts of registration errors in the both groups. The image forming positions are corrected by offsetting the amounts of registration errors. In this way, errors in detecting registration errors that occur from meandering of the belt and the like can be reduced, by using the marks in the left and right groups.